1. Field of the Invention
The invention relates to fabrication methods for electronic devices, and in particular to fabrication methods for electronic devices with via through holes and thin film transistor devices.
2. Description of the Related Art
Conventionally, lithographical fabrication of electronic devices with via through holes includes alignment and exposure techniques to define via through hole patterns using a photo mask. The drawbacks of the lithographical fabrication method are that fabrication apparatus and process costs are very high. The laser drilling process for via through hole is commonly used for fabrication of semiconductor package insulation films. However, laser beams can damage the insulation film by burning nearby resin, thus resolution of laser drilling is limited to about 50 μm and fabrication cost thereof is relatively high. Meanwhile, other conventional techniques such as direct coating of conductive vias using an inkjet printing technique use no optical mask, thus reduces costs. Optical shielding patterns are inkjet printed on predetermined through hole areas, thereby achieving large scale production and being applicable to roll-to-roll processes. Advantages of roll-to-roll processes is the ability to create a drop-on-demand thin film by controlling droplets during inkjet printing, i.e., injecting droplets according to required locations as demanded.
U.S. Pat. No. 7,176,040, the entirety of which is hereby incorporated by reference, discloses a method for forming a via through hole using inkjet printing solvent thereon. Solvent is inkjet printed on the insulation layer. Openings are formed as the solvent etches the insulation layer. Conductive material is then filled into the opening to create conductive via through holes. However, the drawbacks of the abovementioned method are that diameters of the conductive via through holes are determined by surface tension of the solvent and a contact angle between the solvent and the insulation layer. The dimensions of the conductive via through holes are difficult to control. Further, the solvent may damage the underlying layers. The depth of the conductive via through holes must be controlled by the amount of injected solvent droplets. During the etching process for etching openings in the insulation layer, a coffee ring shape structure can be formed at the peripheral of the conductive via through holes, resulting in a bank with different heights. The height differences of the bank may be too high, thus detrimental to subsequent fabrication processes. The shape of the conductive via through hole can be hemispherical due to diameter differences. Additionally, high aspect ratio openings cannot be achieved by injecting solvent droplets. Thus, application of inkjet printing techniques is limited.
In summary, conventional inkjet printing methods for forming conductive via through holes have been disclosed. The depth of the conductive via through holes can be determined by controlling the amount of solvent droplets. However, the diameter, depth, and profile of the conductive via through holes are difficult to control, deteriorating process reliability. Additionally, beside precisely controlling the solvent amount and removing solvent residue, damage to underlying layer structures needed to be overcome to improve upon conventional inkjet printing methods.